RFR: 8353440: Disable FTP fallback for non-local file URLs by default [v11]

[Eirik Bjørsnøs]

> Please help review this PR which disables the unspecified but long-standing feature where an `FtpURLConnection` is opened as a fallback for non-local file URLs.
> 
> Before this change, if a file URL has a non-local host component, say `file://remotehost/folder/data.txt`, then the  implementation would attempt opening an FTP connection to `remotehost`. After this change, such URLs will be rejected with a `MalformedURLException`, unless the FTP fallback feature is explicitly re-enabled via a system property.
> 
> This change was initially discussed here: https://mail.openjdk.org/pipermail/net-dev/2025-March/025988.html
> 
> See the above discussion and CSR draft JDK-8354678 for the motivation for this change.
> 
> This PR:
> 
> * Changes file URL `Handler::openConnection` implementation for unix/windows to throw `MalformedURLException`, unless the FTP fallback feature is explicitly enabled by configuration.
> * Introduces a new system property `jdk.net.file.ftpenabled` which when set to `true` re-enables the feature.
> * Documents the new property in `net-properties.html` 
> * Updates the existing test `NonLocalFtpFallback` to enable the feature via said system property.
> * Adds a new test `NonLocalFtpFallbackDisabled` verifying that a `MalformedURLException` is thrown by default for a non-local URL host component.
> * Moves testing of exceptional behavior in FtpConnection when using non-local file URLs with FTP fallback enabled from `OpenStream` to `NonLocalFtpFallback`
> 
> I have added a Release Note as a subtask in the JBS issue, this also needs a review.

Eirik Bjørsnøs has updated the pull request incrementally with two additional commits since the last revision:

 - Add test for MUE on non-local URL with an empty path component
 - Use Boolean::getBoolean

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/24657/files
  - new: https://git.openjdk.org/jdk/pull/24657/files/af73727f..a5620182

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24657&range=10
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24657&range=09-10

  Stats: 14 lines in 2 files changed: 9 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24657.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24657/head:pull/24657

PR: https://git.openjdk.org/jdk/pull/24657